This invention relates to the field of concrete construction, and more particularly, to devices facilitating the placement of reinforcing dowels within a concrete slab across a control joint.
In the design of concrete slabs formed on a grade it is necessary to account for thermal expansion and shrinkage of the concrete. One way to control the thermal expansion and contraction of concrete within a slab is through the use of an expansion joint. An expansion joint is formed by the placement of compressable materials consisting typically of wood or a composite fiber material at regular intervals throughout the slab. The compressable material allows expansion of the concrete, after curing, beyond its original volume. A second such joint that controls contraction due to loss of moisture during curing is a control joint. Control joints are more prevalent and must be placed every 100 to 200 square feet of surface area.
Generally speaking, a control joint is a cut in a concrete slab which allows the slab to contract during the curing phase of a concrete slab and expand or contract thereafter without damaging the slab. While a control joint protects the concrete slab from damage due to contraction of the concrete during curing and expansion or contraction thereafter within the slab itself, the joints prohibit loads placed on the concrete slab to be transferred uniformly throughout. As a result, control joints, without further reinforcement, leave the concrete slabs susceptible to damage. For instance, loads developed by a forklift moving across a non-reinforced control joint poses a serious risk of damage to the slab because the forklift""s load at the control joint is supported by only one half of the volume of concrete as available to support the forklift in an portion of the slab without a control joint.
A way to prevent buckling or angular displacement of such control joints is to insert smooth steel dowel rods generally known as xe2x80x9cslip dowelsxe2x80x9d within the edge portions of adjoining concrete slabs in such a manner that the concrete slabs may slide freely along one or more of the slip dowels. The slip dowels permit linear expansion and contraction of the slabs and maintain the slabs in a common plane, thus preventing undesirable buckling of the control joint. In order to function effectively, slip dowels must be accurately positioned parallel within the adjoining concrete slabs. Nonparallel positioning of the dowels often prohibits the dowels from sliding within the concrete slab, thereby defeating the purpose of the xe2x80x9cslip dowelxe2x80x9d application.
Traditionally, two methods of installing smooth xe2x80x9cslip dowelsxe2x80x9d have become popular. According to the first method, a first concrete pour is made within a preexisting form. After the pour has cured, holes are drilled horizontally into the slab at corresponding intervals. The depth and diameter of the individual holes varies depending on the application and the relative size of the concrete slabs being formed. As a general rule, however, such holes are at least twelve inches deep and typically have a diameter of approximately five-eighths of an inch. After the parallel aligned series of holes have been drilled into the first pour, smooth dowel rods are advanced into each hole such that one end of each dowel rod is positioned within the first pour and the remainder of each dowel rod extends into the second formed area. Thereafter, concrete is poured into the second area and is permitted to set around the dowels. Once the concrete in the second pour has cured, the dowels are fixedly contained within the second pour and slideably contained within the first pour. Thus, the dowels are capable of transferring loads between the slabs while allowing the slabs to move along the longitudinal axes of the dowels.
While it has become popular to use this method, it is riddled with inefficiencies. First, the method requires the time intensive step of drilling multiple holes within the first cured slab. Second, the holes must be drilled precisely perpendicular to the surface of the control joint in order to ensure that the dowels will permit the concrete slabs to move longitudinally in relation to each other. In practice, such precision is difficult to achieve. Thus, this method has been proven to be an inefficient design.
Another popular method of locating slip dowels within a concrete slab has involved placing a material over one end of a dowel during the construction process, thereby enabling the dowel to slide longitudinally once the concrete has cured. This method has used wax treated cardboard sleeves, plastic sleeves, or grease alone over one end of each individual dowel. According to such method, a series of holes are drilled through one edge of a concrete form and smooth dowels are advanced through each such hole. Wax treated cardboard sleeves, plastic sleeves, or grease alone are placed over one end of each dowel and the first pour is made within the form. After the first pour has set, the previously drilled form is stripped away leaving the individual dowels extending into a second formed space where the second pour is to be made. Subsequently, the second pour is made and permitted to cure. Thereafter, the slip dowels are firmly held by the concrete of the second pour but are permitted to move longitudinally against the inner surfaces of the wax treated cardboard sleeves, plastic sleeves, or grease alone within the first pour. Thus, the wax cardboard sleeves, plastic sleeves, or grease alone allow for longitudinal slippage of the dowels while maintaining the two adjoining concrete slabs in a common plane.
While the second method has alleviated the need for drilling holes in a cured slab, it is nonetheless associated with numerous deficiencies. For instance, the second method requires that the dowels be inserted during the first pour, thereby requiring that the dowels be in place until the second pour is made. A common occurrence at this stage is for the dowels to become bent due to various actions taken at the construction site, thereby making the removal of the concrete form difficult. Additionally, the form provided no means for accurately determining whether the dowel was in correct perpendicular alignment with the concrete slab. Still yet, the system requires the removal of the concrete form prior to pouring the second area.
Although many ideas have been proposed to eliminate these problems, none have sufficiently solved these problems to date. Thus, there exists a need in the art for a devise and method for facilitating the proper placement of slip dowels while insuring proper load transfer across a control joint. Further, a need exists for increasing the efficiency of the installation process.
Set forth below is a brief summary of the invention which solves the foregoing problems and provides benefits and advantages in accordance with the purposes of the present invention as embodied and broadly described herein.
Generally speaking, in a preferred embodiment of this invention, an expendable concrete form is used to form joints within a concrete slab in order to prevent adjoining concrete slabs from cracking due to shrinkage during curing of the concrete slab. The concrete form is composed of a non-typical shape that enables each slab adjoining the concrete form to transfer loads placed on one slab to an adjoining slab in order to prevent cracking and the development of uneven slabs. Additionally, loads are transferred from one slab to an adjoining slab through dowel bars that are inserted into apertures found within the concrete form. Thus, loads are transferred between adjoining concrete slabs through dowel bars and the interlocking slab design that are created through the use of the concrete form.
The apertures, which are preferably square or rectangular, are located on a central member of the concrete form in order to position the dowel bar holder centrally within the concrete slab. The apertures are sized to receive the retention clips of the dowel bar holders. Each dowel bar holder is sized to receive a dowel bar. The dowel bar holders are composed of an open proximal end, a closed distal end, a hollow interior compartment, a plurality of internal fins, and a base member. The internal fins are attached to the interior surfaces of the vertical walls within the hollow interior compartment and operate to position the dowel bar centrally within the dowel bar holder once the dowel bar has been inserted into the dowel bar holder. The base member of the dowel bar holder is composed of a flange, a sleeve, and a plurality of retention clips. The flange further is composed of a plurality of wings.
In operation, a concrete slab is formed by first locating the concrete form in its preferred position with ground stakes. The dowel bar holder is affixed to the concrete form by inserting the retention clips of the dowel bar holder into the aperture a sufficient distance to allow the retention clips to bear against the back surface of the central member of the concrete form. Both the retention clips and the wings of the flange act to positively position the dowel bar holder in a position perpendicular to the front surface of the central member of the concrete form. This process is repeated as many times as is necessary to connect the desired number of dowel bar holders to the concrete form.
Once the dowel bar holders are positively positioned within the concrete form, the dowel bars are inserted into the dowel bar holders, preferably until each dowel bar comes to rest against the closed distal end of each dowel bar holder. Once assembly of the dowel placement apparatus is complete, the first pour area is filled with concrete. Immediately thereafter, or concurrently therewith, the second pour area is filled with concrete and allowed to cure. Once both slabs have cured, the dowel bars and the dowel bar holders are contained within the slabs. In this position, the dowel bar located with the second poured area is in direct contact with the concrete and thereby prevented from moving. However, the portion of the dowel bar housed within the dowel bar holder is not directly contacted by the concrete. Thus, this portion of the dowel bar is able to move along its longitudinal axis.
The upper and lower interior walls of the dowel bar holder prohibit the dowel bars from vertical movement. However, the dowel bars may move laterally if the dowels accept a load greater than the strength of the internal fins located in the interior walls of the dowel bar holder. Should the slabs move parallel to the control joint, the internal fins will resist the movement to a point that is slightly less than the yield point of the concrete. Once the resistance of the internal fins is overcome, the internal fins deform and allow the slabs to move along the control joint prior to the concrete cracking. Thus, the internal fins protect the concrete slabs from cracking.
An object of this invention includes an expendable form which allows for both sides of the concrete pour area to be poured at the same time rather than pouring one area, stripping the form, and subsequently pouring the second area; thereby greatly increasing the time efficiency of pouring a concrete slab.
Another object of this present invention is the superior load handling capabilities found within a square or rectangular dowel.
Yet another aspect of this invention is the use of rectangular apertures within the channel of the concrete form which allows for quick connection of the dowel holders to the expendable concrete form.